Over the past several years, we have been studying two phenomena in cloned populations of CD4 positive T lymphocytes referred to as costimulation and anergy. Both affect the production of the T cell growth factor interleukin-2 (IL-2) produced by these cells. Costimulation entails a 30 to 100-fold enhancement of IL- 2 production when signaling through the antigen-specific T cell receptor is supplemented with signaling through the CD28 receptor on the same cell. Anergy is an anti-proliferative state that the T cell enters when it only receives a signal through the antigen-specific receptor. In this case, subsequent stimulation of IL-2 production is inhibited 10-50-fold. Our goals are to try and understand the molecular mechanisms behind these two phenomena and to explore their relevance in vivo. Recently, we have set up a new model for studying tolerance to persistent low dose antigen in vivo, which results in the generation of a large number of anergic T cells. We call this state adaptive tolerance. We inject CD4+, cytochrome c-specific T cells from a T cell receptor transgenic mouse on a Rag2-/- background (a monospecific T cell population) into a second transgenic mouse (RO) expressing the cytochrome c antigen under the control of the MHC class I promoter and an immunoglobin heavy chain enhancer. Within 24 hours after transfer, the T cells are all activated by the antigen (as evidenced by an increase in size and expression of CD69), and proliferate extensively for several days, increasing in number about 100-fold. This expansion is followed by a deletional phase during which 50% of the cells disappear. Finally, the population reaches a steady state level in which the cells appear to be refractory to restimulation in vivo and in vitro. In this adaptive tolerant state, cytokine responses to high doses of antigen in vitro are inhibited 90%. Expression on restimulation of early activation markers, such as CD69 and CD25, is also greatly impaired, and biochemical studies of signal transduction show an inhibition of Zap70 activation and calcium mobilization following T cell receptor engagement. In vivo Budr labeling showed a slow T cell turnover of about 5% per day. This hyporesponsive state is reversible if the cells are transferred again into a second host not expressing the antigen. Interestingly, if the retransfer is into a host expressing the antigen, the cells remain hyporesponsive and slowly decrease their IL-2 and IFN gamma production by another 6-10 fold over 3-4 weeks. This deeper state of anergy suggested that the tolerance process is adaptable to different levels. To test this idea more directly, we studied a second transgenic mouse (SPK) expressing about a 4 fold lower amount of antigen. TCR transgenic CD4 T cells injected into this mouse also proliferated and entered into an anergic state, although with slower kinetics. Eventually, however, the turn over rate in vivo was comparable to that of T cells in the RO environment. Interestingly, in vivo restimulation on retransfer into a fresh RO mouse revealed a greater impairment in the proliferative ability of T cells resident in a higher antigen presentation environment. We also observed subtle differences in TCR signaling and in vitro cytokine production consistent with differential adaptation. Unexpectedly, the system failed to similarly compensate to the persistent stimulus in vivo at the level of CD69 expression and actin polymerization. This greater responsiveness of T cells residing in a host with a lower level of antigen presentation allowed us to demonstarte for the first time an intrinsic "tuning" process in mature T lymphocytes, although one more complex than current theoretical tuning models would have predicted. In an independent study in the lab on the role of DNA demethylation in transcriptional regulation of the IL-2 gene, we located a small 600 bp region in the promoter/enhancer of the gene that demethylates in T lymphocytes following activation, and remains demethylated thereafter. This epigenetic change was necessary and sufficient to enhance transcription in reporter plasmids. The demethylation process started as early as 20 minutes after stimulation in vivo and was not prevented by a G1 to S phase cell cycle inhibitor (Rapamycin) that blocks DNA replication. These results imply that the demethylation process proceeds by an active enzymatic mechanism and suggests the existence of a site-specific demethylase. We speculate that the major function of this event is to enhance the amount or rate of transcription of the IL-2 gene in a memory T cell population to facilitate the rapid production of this cytokine in a secondary immune response.